The LCD cell according to the invention is of very similar construction to known cells, such as twisted nematic LCD's. Schadt and Helfrich, Appl. Phys, Lett. 18, 127 (1971).
It has been known for some time that LCD cells having highly twisted nematic liquid crystals, despite their intensive interference colours in the off state, have advantages over similar cells having a 90.degree. twist. The term "highly twisted nematic structures" is to be understood as denoting structures having a twist of more than 90.degree.. Doping with chiral additives is usually necessary to produce such a twist as is known in the art.
The main advantages of liquid crystal cells having highly twisted nematic structures are a very steep electro-oPtical characteristic and a wide viewing angle range. The steep electro-optical characteristic results in high multiPlex ratios which are necessary to provide displays of high information density.
Apart from earlier ProPosals for high-twist liquid crystal cells based on the guest-host effect, one of the first high-twist nematic liquid crystal cells without any colouring additive was the one disclosed in European Patent Application No. 131 216 of Amstutz. H. et al. The liquid crystal of this cell has a twist of from 180.degree. to 360.degree..
The polarizers are offset at angles of from 20.degree. to 70.degree. relative to the orientation directions on the screens. The ratio d/p between the layer thickness (d) and pitch (P) of the liquid crystal is between 0.5 and 0.95. The cell requires a sPecial wall orientation on the electrode surfaces, leading to high tilt angles of the nematic director. Wall orientations of this kind are complex to produce and therefore expensive.
Subsequently, increasing work was done in various places on the develoPment of liquid crystal cells having high-twist nematic structures, because LCDs are being increasingly used in large-area high-density-information applications and high multiplexibility is essential for this Particular use. Thus, many angle ranges between 90.degree. and 360.degree. for the twist and for the arrangement of the polarizers relative to the surface orientations and to one another and for the proportions of the optical anisotropy .DELTA.n, the electrode distance or thickness d and the natural pitch p of the chirally doped nematic liquid crystal relative to one another have been suggested. Various advantages have been alleged for each of the disclosed high-twist structures, with the emphasis on improved multiplexibility and improved range of viewing angles. A disadvantage was found to be the intensive interference colours of such display cells which, if colour variations are to be avoided, call for very accurate plate spacing and, therefore, exPensive and highly precise production processes. Also, the strong inherent colouring of such disPlays is an obstacle to their use as neutral-colour electro-oPtical switches such as are required inter alia for colour TV or for coloured display screens.
The Problem which it is the object of the invention to solve arises from the Present need for even better LCD cells with more particularly less inherent colouring, the possibility of producing grey tones, simple production. short switching times together with high optical contrast, reduced dePendence uPon viewing angle, good multiplexibility and a display Picture which is relatively unaffected by variations in the plate sPacing of the cell.